Hi Miguel,
the behavior you are observing is common for all systems. I.e 32-bit Linux, 64 bit Mac OS, 64 bit Linux. Could you please be so kind so try and run the following on 64 bit Linux?
CREATE TABLE t1 ( a INT NOT NULL, KEY(a) );
INSERT INTO t1 VALUES (1), (2), (3);
EXPLAIN SELECT MIN( a ) FROM t1 WHERE a <=> NULL;
SELECT MIN( a ) FROM t1 WHERE a <=> NULL;
EXPLAIN SELECT MIN( a ) FROM t1 WHERE a <=> NULL;
Please be sure to include both EXPLAIN's. This would be very helpful to me.
Thank you.